kmdb

Synopsis

Boot-time Loading

SPARC

ok boot [device-specifier] -k [-d] [boot-flags]

ok boot [device-specifier] kmdb [-d] [boot-flags]

x86

kernel$/platform/i86pc/kernel/$ISADIR/unix-k [-d] [boot-flags]

Runtime Loading

mdb-K

Description

kmdb is an interactive kernel debugger which implements the user interface and
functionality of mdb(1) in a live kernel context. kmdb provides features that allow
for the control of kernel execution and for the inspection and modification
of live kernel state. kmdb can be loaded at the beginning of
a boot session or after the system is booted.

This man page describes the features and functionality that are unique to
kmdb or different in kmdb as compared to mdb(1). For more information on
mdb(1) or further details on the features and functionality implemented by kmdb,
see the mdb(1) man page and the Oracle Solaris Modular Debugger Guide.

Loading and Unloading

Boot-time Loading

When requested, the kernel runtime linker (krtld) loads kmdb prior to the transfer of control to the kernel. If the -d flag is used, the debugger gains control of the system prior to the execution of the initial function in the unix object. If -d is not used, kmdb is loaded but does not gain control until such time as it is explicitly entered. See the Debugger Entry section below. For a list of the boot commands which cause kmdb to be loaded at boot, see the SYNOPSIS section above. See eeprom(1M) for an example of the use of that command on a SPARC machine to specify that kmbd is always loaded upon boot.

Boot-loaded kmdb can be unloaded only by means of a system reboot.

Some features of kmdb rely on the presence of kernel services and are not immediately available to boot-loaded kmdb. In particular, the loading and unloading of dmods is not available until the module subsystem is initialized. Requests are queued until they can be processed. Similarly, translation of virtual addresses to physical addresses is not be available until the VM system has been initialized. Attempted translations fail until translation facilities are available.

Run-time Loading

kmdb can also be loaded after the system has booted, using the -K flag to mdb(1). When loaded in this fashion, it will immediately gain control of the system. Run-time-loaded kmdb can be unloaded using the -U flag to mdb(1) or from within the debugger with the -u flag to the ::quit dcmd.

Terminal types

When loaded, kmdb attempts to determine the proper terminal type in use on the system console. If the system being debugged has an attached keyboard and local display that are both used for the system console, kmdb uses the terminal type appropriate for the machine: 'sun' for SPARC; 'sun-color' for x86. When a serial console is in use, boot-loaded kmdb defaults to a terminal type 'vt100'. Run-time-loaded kmdb defaults to the terminal type requested by mdb(1). mdb(1) requests the terminal type specified by the value of the TERM environment variable unless overridden by the -T flag. ::term can be used to view the current terminal type.

Debugger Entry

Debugger entry can be requested explicitly or implicitly. Implicit entry, encountered when
breakpoints or other execution control features are used, is discussed in the
Execution Control section.

The primary means for explicit debugger entry is with the keyboard abort
sequence for systems with local consoles and the BREAK character for those
with serial consoles. The abort sequence is STOP-A or Shift-Pause for SPARC
systems with local consoles, and F1–A or Shift-Pause for x86 systems with local
consoles. See kbd(1) for a discussion of the abort sequence and for instructions
on disabling it.

A second way to request entry into the debugger is with the
mdb(1) command. Invocations of mdb(1) with the -K flag after the debugger
is loaded trigger debugger entry.

If the kernel panics and kmdb is loaded, by default, the panic
routine enters kmdb for live debugging. If a dump device is specified,
and you enter ::cont, the debugger exits and a crash dump is
performed. To prevent the kernel from entering kmdb when panicking, you can set
the nopanicdebug variable to 1. Set the nopanicdebug variable to 1 using
kmdb or including the following a line in /etc/system:

set nopanicdebug = 1

This can be useful if you want to keep kmdb loaded, but
always want a panic to trigger a crash dump without entering the
debugger.

Execution Control

For the most part, the execution control facilities provided by kmdb for
the kernel mirror those provided by the mdb(1) process target. Breakpoints (::bp), watchpoints
(::wp), ::continue, and the various flavors of ::step can be used.

In contrast to the unlimited user process watchpoints supplied by the kernel,
kmdb is restricted to a set of CPU watchpoints that limit the
number, size, and type of watchpoints allowed. The ::wp command does not
allow a watchpoint to be created if it is incompatible with the watchpoints
supported by the hardware.

Debugger modules (dmods)

As with mdb(1), kmdb is installed with a number of subsystem-specific debugger
modules, or dmods. The dmods are loaded and unloaded automatically with the loading
and unloading of the subsystems that they support. The dmods can also
be explicitly loaded and unloaded using ::load and ::unload.

kmdb uses kernel facilities to load and unload dmods and must resume
system execution to perform each requested action. When a dmod load or
unload is complete, the system is stopped and the debugger is automatically
re-entered. For a dmod load, processing is completed when the load of a
requested dmod succeeds or fails. Status messages are provided in either case.

Processor-specific functionality

Some functionality is specific to an individual processor type. An example of
such functionality is the branch tracing provided by various x86 processors. Access
to these processor-specific features is provided with processor-specific dcmds that are present
only on systems that support them. The availability of processor-specific support is indicated
in the output of the ::status dcmd. The debugger relies on the kernel
to determine the processor type. Even though the debugger might provide support
for a given processor type, the support is not exposed until the kernel
has progressed to the point at which processor identification has completed.

Kernel Macros

The debugger provides access to a set of macros that are precompiled
into the debugger. Only the precompiled macros are available . Unlike with
mdb(1), the $< dcmd may not be used to load macros from arbitrary
locations. Use the $M command to list the available macros.

Built-in dcmds

This section lists dcmds that are unique to kmdb or those with
behavior that differs in kmdb as compared to mdb(1).

[address]::bp [+/-dDestT] [-ccmd] [-ncount] sym ...

address:b [cmd...]

Set a breakpoint at the specified locations. The ::bp dcmd sets a breakpoint at each address or symbol specified, including an optional address specified by an explicit expression preceding the dcmd, and each string or immediate value following the dcmd. The arguments can be symbol names or immediate values denoting a particular virtual address of interest.

If a symbol name is specified, the name may refer to a symbol that cannot yet be evaluated. It might consist of an object name and function name in a load object that has not yet been opened. In such a case, the breakpoint is deferred and is not active in the target until an object matching the given name is loaded. The breakpoint is automatically enabled when the load object is opened.

The -d, -D, -e, -s, -t, -T, -c, and -n options have the same meaning as they do for the ::evset dcmd. See mdb(1) for a description of ::evset. If the :b form of the dcmd is used, a breakpoint is set only at the virtual address specified by the expression preceding the dcmd. The arguments following the :b dcmd are concatenated together to form the callback string. If this string contains meta-characters, it must be quoted.

::branches [-v]

(x86 only)

Display the last branches taken by the CPU. This dcmd is supported only on x86 systems, and is available only when processor-specific support is detected and enabled. The number and type of branches displayed is dependent on the capabilities of the branch tracing facilities provided by the CPU. When the -v option is used, the instructions prior to a given branch are displayed.

[function] ::call [arg [arg ...]]

Call the specified function using the specified arguments. The called function must be listed as a function in the symbol table for a loaded module. String arguments are passed by reference. When the call completes, the return value of the function is displayed.

This dcmd must be used with extreme caution. The kernel will not be resumed when the call is made. The function being called may not make any assumptions regarding the availability of any kernel services, and must not perform operations or calls that may block. The user must also beware of any side-effects introduced by the called function, as kernel stability might be affected.

[addr] ::cpuregs [-ccpuid]

Display the current general purpose register set for the specified CPU, in the format used by ::regs.

[addr] ::cpustack [-ccpuid]

Print a C stack backtrace for the specified CPU. The backtrace displayed is for the point at which the specified CPU entered or was stopped by the debugger.

addr[,len] ::in [-Llen]

(x86 only)

Read len bytes from the I/O port specified by addr. The value of the -L option, if provided, takes precedence over the value of the repeat count. The read length must be 1, 2, or 4 bytes, and the port address must have the same alignment as the length.

addr[,len] ::out [-Llen] value

(x86 only)

Write value to the len-byte I/O port specified by addr. The value of the -L option, if provided, takes precedence over the value of the repeat count. The write length must be 1, 2, or 4 bytes and the port address must have the same alignment as the length.

::quit [-u]

$q

Causes the debugger to exit. When the -u option is used, the system is resumed and the debugger is unloaded. The -u option may not be used if the debugger was loaded at boot. When the -u option is not used, SPARC systems will exit to the boot PROM ok prompt. The go command can be used to re-enter the debugger. On x86 systems, a prompt is displayed that requests permission to reboot the machine.

::step [over|out|branch]

Step the target one instruction. The optional over argument is used to step over subroutine calls. When the optional out argument is specified, the target program continues until control returns from the current function.

The optional branch argument is available only on x86 systems when processor-specific support is detected and enabled. When ::step branch is specified, the target program continues until the next branching instruction is encountered.

On SPARC systems, the ::step dcmd may not be used to step 'ta' instructions. Similarly, it may not be used on x86 systems to step 'int' instructions. If the step results in a trap that cannot be resolved by the debugger, a message to that effect is printed and the step will fail.

cpuid::switch

cpuid:x

Use the specified CPU as the representative. Stack traces, general purpose register dumps, and similar functionality use the new representative CPU as the data source. Full execution control functionality is available on the new representative CPU.

::term

Display the current terminal type.

addr[,len]::wp [+/-dDestT] [-rwx] [-pi] [-ncount] [-ccmd]

addr[,len]:a [cmd...]

addr[,len]:p [cmd...]

addr[,len]:w [cmd...]

Set a watchpoint at the specified address, interpreted by default as a virtual address. If the -p option is used, the address is interpreted as a physical address. On x86 platforms, watchpoints can be set on I/O ports using the -i option. When the -i option is used, the address is interpreted as that of an I/O port.

The length in bytes of the watched region can be set by specifying an optional repeat count preceding the dcmd. If no length is explicitly set, the default is one byte. The ::wp dcmd allows the watchpoint to be configured to trigger on any combination of read (-r option), write (-w option), or execute (-x option) access.

The -d, -D, -e, -s, -t, -T, -c, and -n options have the same meaning as they do for the ::evset dcmd. See mdb(1) for a description of ::evset. The :a dcmd sets a read access watchpoint at the specified address. The :p dcmd sets an execute access watchpoint at the specified address. The :w dcmd sets a write access watchpoint at the specified address. The arguments following the :a, :p, and :w dcmds are concatenated together to form the callback string. If the string contains meta-characters, it must be quoted.

See Also

SPARC Only

Notes

Limitations on Memory Available to the Debugger

The memory region available to the debugger is allocated when the debugger
is loaded, and is fixed at that point. If dcmds attempt to
allocate more memory than is available, they will, if possible, be terminated.
The debugger will attempt to recover gracefully from an out-of-memory situation, but may
be unable to, and may be forced to terminate the system. This
constraint is especially acute on 32-bit x86 systems.

Performance Impact

System performance will be negatively impacted by the loading of kmdb, as
the debugger will consume kernel memory and other limited system resources.

Booting into kmdb to Capture panic() Stack

To troubleshoot a panic() on a SPARC machine, it can be useful
to use eeprom(1M) to specify that the system always load kmdb upon booting.
Following a panic, the system starts to reboot, in so doing clearing
the panic stack from the console. By booting into kmdb, one can
capture and interpret the panic stack. See eeprom(1M) for an example of
specifying that kmdb load upon boot.